In some gas sensor applications, it is necessary before the measurement to convert the gas analyte into another product that is detectable by the sensor. Reasons for this can e.g. lie in the characteristic of the sensor, if it cannot detect the original target gas, or e.g. lie in increasing the specificity of the measurement, if the specificity of a sensor is lower for the original target gas than the specificity of the same (or another) sensor for the converted target gas. It is precisely in the detection of individual gasses in a complex gas mixture (such as the human exhaled air) that the specificity or cross-sensitivity of the sensor plays a decisive role.
Very small amounts of nitric oxide (nitrogen monoxide, NO) are continuously released into the respiratory gas flow from the cells of the airways. NO constitutes an important marker for the diagnosis and optimized therapy of asthma and other inflammatory airway diseases. With a prevalence of approximately 5% of adults and approximately 20% of children in developed industrial nations, asthma is one of the most prevalent diseases. In inflammatory processes of the airways, e.g. asthma, there are elevated NO concentrations of 40 ppb (parts per billion) and more in the exhaled air. Imminent asthma attacks can be recognized significantly earlier from an increase in the NO content of the exhaled air than from a lung-function test, which ultimately only quantifies a symptomatic narrowing of the bronchi. Thus, the NO measurement in the exhaled air is a preferred method for diagnosing and monitoring the therapy progress of asthma and other inflammatory airway diseases.
Cost-effective NO sensors with the required sensitivity in the ppb range have until now not been commercially available. A newly developed NO2 sensor based on “suspended gate FET” technology meets the aforementioned requirements. However, a conversion module for converting the NO in the respiratory gas into NO2, which can be detected by the sensor, must be placed upstream of such a sensor. Ideally, such a conversion module should hold for a number of months or even years, be cost-effective and convert NO into NO2 at a very high conversion rate, which is constant.
Nitrogen monoxide is converted to nitrogen dioxide as per the following reaction equation:2NO+O22NO2.
Nitrogen monoxide can be converted to nitrogen dioxide in a respiratory gas sensor instrument by an arrangement for oxidizing nitrogen monoxide to nitrogen dioxide, for example by guiding the (respiratory) air through an oxidizing agent (e.g. potassium permanganate, perchlorate salts or the like) or another oxidation catalyst.
The fact that NO2 is significantly more soluble in water than NO constitutes another problem. Therefore, a method is required for keeping the concentration of the converted NO2 as constant as possible and quantitatively measurable in the moist respiratory gas. As a result of the higher solubility of NO2 in water, part of the (converted) NO2 is dissolved in water in (respiratory) air with a high moisture content, the concentration of the measurable NO2 drops and a seemingly too low NO2 and, respectively, NO content is measured.
If it is possible to convert the NO in the respiratory air into NO2 in a reliable and quantitative fashion, this sensor can be used to measure the NO in asthma patients. Ideally, such a conversion module should hold for a number of months or even years, be cost-effective and convert the NO into NO2 at the highest conversion rate, which is as constant as possible. The challenge in optimizing the gas conversion is that, on the one hand, a long retention time of the gas in the arrangement for gas conversion improves the conversion rate and, on the other hand, already generated NO2 is increasingly adsorbed in the arrangement for gas conversion in the case of a long retention time. Additionally, a long conversion path increases the respiratory resistance against which the patient must exhale. Since an excessive respiratory resistance is inadmissible for the measurement of the target gas (see: ATS/ERS Recommendations for Standardized Procedures for the Online and Offline Measurement of exhaled Lower Respiratory Nitric Oxide and Nasal Nitric Oxide, 2005; American Thoracic Society (ATS) and the European Respiratory Society (ERS)), an upper limit is prescribed hereby for the length of the conversion path.
A further problem lies in the fact that the conversion rate of an arrangement for gas conversion is not constant over the service life, but decreases with time. This leads to an error in determining the concentration of the initial gas.